With a PD, the vibration is detected as the light output variation caused by the optical length difference between the MC and another LD LD1.. This sideways vibration is detected by the L
Trang 10.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17
0 0.02 0.04 0.06 0.08 0.1
Effective reflectivity
(b) 0.18
(a)
Effective reflectivity
0 1 2 3 4 5 6 7
0 0.02 0.04 0.06 0.08 0.1 8
Fig 2.22 Dependence of side-mode suppression ratio on Reff2 (a), and dependence
of spectrum line width on Reff
2 (b).
2.4.1 Tunable LD
A vertical cavity surface-emittinglaser (VCSEL) diode or a light-emitting diode (LED) with a micromechanical reflector can be used in tuned de-vices [2.4, 2.5] The structure is designed to have an air gap of approximately one wavelength When a voltage is applied to the membrane reflector, the electrostatic force reduces the air gap, which in turn reduces the wavelength
An edge-emitting LD is also applicable for micromechanically tunable LDs [2.7] Recent micromachiningtechnology has made it easy to eliminate the need for lens and fiber systems for guiding the light to a PD or a moving mechanism, leadingto the integration of optics, mechanics, and electronics
Structure
The edge-emitting tunable laser diode consists of a laser diode LD1, a microcantilever MC driven photothermally by an LD2 (Fig 1.28) The light emitted from LD2 onto the side wall of the MC is partially absorbed, heating
Trang 2the MC and producingthe bendingmoment At resonant frequency, the MC
is excited easily due to the thermal stress caused by a pulsed laser beam from
LD2 This sideways vibration varies the external-cavity-length Lex between the MC wall and the LD1 facet, and there is so little incident light from the
LD1 that it has no effect on the MC vibration The variation of Lex causes the wavelength shift of the LD1
Manufacturing Method
An MC and the LDs were fabricated on a GaAs substrate There are three micromachiningprocesses involved in fabricatingthe MC (1) an etch-stop layer of AlGaAs is formed in the LD structure prepared by metalorganic vapor phase epitaxy (MOVPE).(2) The microstructure shape is precisely defined by
a reactive dry-etching(RIBE) technique, which can simultaneously form the vertical etched mirror facets for LDs (3) A wet-etch window is formed with photoresist and the MC is undercut by selective etchingto leave the MC freely suspended (Fig 1.11) [2.19]
These processes are compatible with laser fabrication, and thus an MC structure can be fabricated at the same time as an LD structure Furthermore, because a single crystal epitaxial layer carries little residual stress, precise mi-crostructures can be obtained without significant deformation We fabricated
an MC with an area of 400× 700 µm The MC was 3 µm wide, 5µm hig h and
110µm long The shorter the MC–LD2 distance becomes, the higher the pho-tothermal conversion efficiency The threshold current of the LD was 46 mA Figure 2.23 shows the main parts of the tunable LD The hole for wet etching
is visible under the MC between LD1 and LD2
Monolithic integration of optics and micromechanics is possible not only
on a gallium arsenide (GaAs) substrate [2.19], but also on an indium phos-phide (InP) substrate [2.20,2.21] A smooth, etched surface and a deep vertical sidewall are necessary for good lasing characteristics of both types of semi-conductor microstructures
Basic Characteristics
In a micromechanically tunable LD, the movingpart (MC) was integrated with an edge-emitting LD By varying the external cavity length (MC deflec-tion), the laser wavelength can be easily changed and the wavelength shift
varied every half-wavelength (λ/2) Therefore, the MC must move more than λ/2 even at off-resonant frequencies In Sect 2.5.1, we present the design of the MC structure that satisfies photothermal deflection of greater than λ/2.
We have experimentally analyzed how the parameters of the couplingsys-tem affect the ESEC LD operation by usinga rotatingoptical disk and an LD attached to a flyingslider The parameters included the reflectivities of the
LD facets, the reflectivity of the external mirror, and the LD drive current
We confirmed a 30 nm tuning range around a wavelength of 1.3µm, as shown
Trang 3Microcantilever (MC)
Laser diode (LD1)
Laser diode (LD2)
GaAs
Fig 2.23 Scanning electron microscope view of the main parts of the tunable LD.
The released GaAs/AlGaAs microcantilever (MC)was fabricated by undercutting the sacrificial GaAs The MC length, thickness and width are 110, 3, and 5µm and the distances from the facet of LD1 to the side wall of the MC and LD2 to MC are
3 and 30µm, respectively Courtesy of O Ohguchi, NTT, Japan
in Fig 2.16, by changing the external-cavity length for the LD with an antire-flection coatingon the facet facingthe external mirror On the basis of these results, we consider that by employingthe MC design and the fabrication method described earlier, a photothermally driven micromechanical tunable
LD will be available in the future
2.4.2 Resonant Sensor
A resonant sensor is a device that changes its mechanical resonant frequency
as a function of a physical or chemical parameter, such as stress or mass-loading[2.22] Electrostatic (capacitive) excitation and detection or piezo-electric excitation and detection have been used in conventional silicon-based resonant sensors The former method requires comparatively large electrode areas to obtain good signals, which presents difficulties at the microscale The latter requires a layer of a piezoelectric material, preferably zinc oxide (ZnO) However, unfortunately, ZnO is not compatible with integration technology
Structure
A resonant MC, LDs, and a PD have been fabricated on the surface of a GaAs substrate, as shown in Fig 2.24 The MC is excited photothermally by light from one laser diode (LD2) With a PD, the vibration is detected as the light output variation caused by the optical length difference between the MC and another LD (LD1)
The resonator was designed to optimize the efficiency of the photothermal excitation and the quality of the composite cavity signal with the structural
Trang 4Microcontilever
Fig 2.24 Photograph of a resonant sensor with a MC driven photothermally from
one side by LD2 and sensed optically from the other side by LD1 and photodiode (PD1); LD2, MC, LD1, and PD1 are integrated on a GaAs substrate
configuration resulting from the fabrication process The distance h1 between
the facet of LD1 and the wall of the MC was set to 3.0µm, on the basis of
the composite cavity signal SNR and the aspect ratio h1/w of the reactive dry-etchingprocess The distance h2 between the facet of LD2 and the wall
of the MC was set to be 30µm considering the energizing light absorption
on the MC, and the hole size for the wet process described later The MC
dimensions were set to a length l = 50 µm and 110 µm, a thickness t = 3 µm, and a width w = 5µm, consideringthe resonant frequency of the MC The positions of the excitation light (LD2) and the detection light (LD1) on the
MC wall were chosen consideringthat the LD2 light strikes the MC closer to the support for better excitation, and that the LD1 light strikes further from the support for better detection as well as to prevent cross-talk between the two light beams
The short distances in the LD2–MC–LD1–PD structure are useful for a vibration resonator because no lenses are required between LD1, MC, and LD2 to make the light beam converge, so it is easier to integrate the mechan-ical element with the optmechan-ical elements Furthermore, the integrated structure does not need any optical alignment like that required by conventional hybrid resonant sensors
Basic Characteristics
The MC is excited by the resonant frequency due to the thermal stress caused
by a pulsed laser beam from LD2 This sideways vibration is detected by the LD1 and the PD from the variation in the external cavity length between the
MC wall and the facet of LD1 (phase difference) Light incident from LD1
is continuous illumination and is so small that it has no effect on the MC
Trang 5Microcantilever (MC)
Laser diode (LD1) h
Light output
Signal
External cavity length (mm)
0
Dh
Dh l/2
Fig 2.25 Maximum peaks in the light output occur every λ/2 and their amplitude
decays exponentially in proportion to the external cavity length
vibration The variation in light output caused by this vibration is detected
as a signal by the PD Maximum peaks in the light output occur every λ/2 and
their amplitude decays exponentially to the external cavity length as shown
in Fig 2.25 The variation in light output caused by this vibration is detected
by the PD
Figure 2.26 shows that the signal amplitude increases as the LD2 light power increases, but an inversion appears in the signal peak for the light
power over 30 mW, because the vibration amplitude is larger than λ/4 We
can determine the absolute amplitude on the basis of the fact that the peak
signal amplitude corresponds to λ/4 (0.21µm) As the incident light power rises, producinggreater thermal expansion(stress) in the MC, the vibration amplitude increases
Figure 2.27 shows a photograph with different excitation light
posi-tions The MC deflections, ∆h1 (detectingside: LD1) and ∆h2 (excitation side: LD2), were measured independently by the method described earlier Figure 2.28 shows the relationship between the deflection and the normalized excitation position with a laser power of 9.5 mW It is confirmed that both deflections increase as the light strikes the MC closer to the support, and
∆h1 is greater than ∆h2, probably due to the optical pressure exerted by the light from LD2
To measure the resonant properties, LD2 was lased by the current with various frequencies When the current frequency coincided with the MC
Trang 6(a) (b)
20 mW
30 mW
40 mW
Fig 2.26 Resonant signal amplitude and spectrum versus LD2 light power The
signal amplitude increases as the light power increases, but an inversion appears in the signal peak for the light power over 30 mW, because the vibration amplitude
becomes larger than λ/4
Laser diode Microcantilever
Illuminated spot
l0
x
Fig 2.27 Photograph of the illuminated spot on the MC, for investigating the
excitation efficiency depending on the position of the MC
mechanical resonant frequency, the amplitude of the LD1 light output ex-hibited a maximum The signal can be obtained from the interference be-tween the LD1 output light and its reflected light from the MC sidewall Figure 2.29 shows the resonant frequency and frequency spectra of the MC The resonances of the MC for lengths of 110µm and 50 µm were 200.6 kHz and 1.006 MHz, respectively They are in good agreement with the theoretical
Trang 7Normalized laser spot position (x / l0)
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Dh 2
Dh 1
Lens
LD1 MB
x
Fig 2.28 Variation in MC vibration amplitude as a function of the illuminated
spot position on the MC
Cantilever microbeam length ( mm)
108
107
106
105
10 4
10 3
102
Frequency (kHz)
Frequency (kHz)
30 25 20 15 10 5 0
990 1000 1010 1020
80
60
40
20
0
190 195 200 205 210
Fig 2.29 Resonance frequency and frequency spectra as a function of MC length
for GaAs
results calculated from (2.27) [2.23, 2.24]
t
E 12ρ
where λ0 is the eigen value of 1.875 determined by the vibration mode, E is Young’s modulus, ρ the density, l the cantilever length, and t its thickness The Q in air were approximately 250 In order to increase Q,
dampingmech-anisms such as imbalance and radiation at the supportingrim require further studies To increase the sensor sensitivity, the resonant frequency should be
Trang 8LD2 LD1
MC
Laser diode (LD1)
Laser diode (LD2)
Microcantilever (MC)
Hole for wet-etching
Fig 2.30 Photograph of a resonant sensor deposited with chemically inductive
material phthalocyanine of 1µm thickness
Fig 2.31 Resonant frequency change of 500 Hz from 288.4 to 287.9 kHz, due to
the mass increase of 54 ng for the 1-µm thick phthalocyanine deposition
increased by shorteningthe cantilever length A resonant frequency of 10 MHz
is applicable with a length of less than 20µm(3 µm thick)
Possible applications are resonant frequency change detection type ac-celerometers and gas sensors Chemically inductive material phthalocyanine was deposited of 1µm thickness on the resonator as shown in Fig 2.30 Then the resonant frequency was changed by 500 Hz from 288.4 to 287.9 kHz due
to the mass increase of 54 ngcorrespondingto the 1-µm thick phthalocya-nine deposition It was confirmed that the resonant sensitivity is very high (Fig 2.31) Both figures show the possibility of detecting a gas
The yield strength of single crystalline GaAs is less than that of Si, but it
is five times greater than that of steel Furthermore, micromachining can be used to fabricate microstructures of high purity with a low defect density and
no residual stress These mechanical properties mean that GaAs-based and InP-based microstructures are suitable for use in integrated micromechanical photonics systems
Trang 92.4.3 Optically Switched Laser Head
In this section a small flyingoptical head and its high quality readout charac-teristics when used as an optically switched laser (OSL) head are described The basic concept involved the use of light emitted and collected through
a 1-µm diameter aperture of an LD placed less than 2 µm from a recording medium (ESEC LD configuration) For autofocus, the head operates like a magnetic head: air-bearing technology stabilizes the slider flying height ap-proximately 1µm as shown in Fig 2.32 Controlled by a sampled servo track error signal, the arm used to seek from track to track is also used for track followingas described later
Drive Structure
A prototype drive consistingof a flyinghead and a phase change medium disk is constructed for experimental purposes Figure 2.33 shows experimental optical disk drive usingan OSL head and a phase change recordingmedium: linear actuator type (a) and rotary actuator type (b)
Slider
Spring Optical Disk
Head Arm
h
PD
LD
Fig 2.32 Schematic representation of an optically switched laser (OSL)head flying
on an optical disk (a), and detailed view of the flying slider on which an LD–PD is mounted (b)
OSL Head
Optical Disk (86 mm)
Optical Disk (50 mmf)
Rot Actuator
OSL Head Linear Actuator
Fig 2.33 Experimental optical disk drive using an OSL flying head and a phase change recording medium: linear actuator type (a), and rotary actuator type (b)
Trang 10An LD monolithically integrated with a PD is mounted junction-up on a slider Light reflects from the medium back into the active region of the LD
Head-medium spacing h (between the LD facet and the GeSbTe recording
medium) is approximately 2µm: the sum of the slider flyingheight h0, LD–
PD attachment error h1, and the protective layer thickness h2
Head Structure
A monolithically integrated LD–PD chip with a wavelength of 1.3µm was shown in Fig 1.33 The LD is isolated from the PD by reactive ion beam etching(RIBE) The space between LD and PD is about 5µm and the monitor current sensitivity is 0.1 mA/mW The 1.2-µm-wide taper-ridged waveguide
on the top of the LD cavity was also fabricated by RIBE FWHM of its near field pattern are approximately 1µm as shown in Fig 2.34 This sharpened LD
is useful for the flyingoptical head because it does not require an additional lens to converge the light beam, and hence does not lose power before reaching the recordingmedium
A long-wavelength (1.3µm) InGaAsP LD (LD#1), reliable in air, can be used in our flyinghead because its spot diameter is mainly constrained by the
shape of the ridged waveguide [2.25] A short-wavelength (0.83µm) GaAlAs
LD (LD#2) could be used if its facets were covered with dielectric protective films to prevent oxidation in air
Medium Structure
The optical disk is made up of multiple layers: SiN/GeSbTe/SiN/Au/SiN/glass substrate as shown in Fig 1.32 The first SiN layer operates as a protective film for a head-medium reliability The GeSbTe layer serves as the phase change medium The second SiN layer and the Au layer enhance the re-flectivity change and the thermal diffusion speed of the recording medium
0.65 mm
0.85 mm
(T ) ( // )
Fig 2.34 Near field pattern of the emitted light from a 1.2-µm wide taper-ridged waveguide